Short-Term Ecogeomorphic Evolution of a Fluvial Delta from Hindcasting Intertidal Marsh-Top Elevations (HIME)
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remote sensing Article Short-Term Ecogeomorphic Evolution of a Fluvial Delta from Hindcasting Intertidal Marsh-Top Elevations (HIME) Brittany C. Smith 1, Kevan B. Moffett 2,* and David Mohrig 3 1 Princeton Hydro, Ringoes, NJ 08551, USA; [email protected] 2 School of the Environment, Washington State University, Vancouver, WA 98686, USA 3 Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712–1722, USA; [email protected] * Correspondence: kevan.moff[email protected] Received: 16 March 2020; Accepted: 29 April 2020; Published: 9 May 2020 Abstract: Understanding how delta islands grow and change at contemporary, interannual timescales remains a key scientific goal and societal need, but the high-resolution, high frequency morphodynamic data that would be most useful for this are as yet logistically prohibitive. The recorded water levels needed for relative elevation analysis are also often lacking. This paper presents a new approach for hindcasting intertidal marsh-top elevations (HIME) to resolve ecogeomorphic change, even in a young, rapidly changing fluvial delta setting, at sub-decadal temporal resolution and at the spatial resolution of widely available optical remote sensing imagery (e.g., 30 m Landsat). The HIME method first calculates: (i) the probability of land exposure in a set of historical imagery from a user-defined discrete timespan (e.g., months or years); (ii) the probability of water level non-exceedance from water level records, which need not be complete nor coincident with the imagery; and (iii) the systematic variation in local mean water level with distance along the primary hydraulic gradient. The HIME method then combines these inputs to estimate a marsh-top elevation map for each historical timespan of interest. The method was developed, validated, applied, and results analyzed to investigate time-lapse evolution of the Wax Lake Delta in Louisiana, USA, every three years, over two decades (1993–2013). The hindcast maps of delta island extents and elevations evidenced ecogeomorphic system self-organization around four stable attractors, or elevation platforms, at about 0.3 m (subtidal), 0.2 m, 0.4 m, and 0.9 m (supratidal) NAVD88. The HIME results also yielded a time − series of net subaerial sediment accumulation, and specific locations and magnitudes of gains and losses, at scales from 30 m to delta-wide (~100 km3) and 6 to 21 years. Average subaerial net sediment accumulation at the Wax Lake Delta (WLD) was estimated as 0.6 cm/yr during the study period. Finally, multiple linear regression models were successfully trained on the HIME elevation maps to model evolving delta island morphologies based on simple geometric factors, such as distance down-delta and position on a delta island; the models also successfully reproduced an average delta topset slope of 1.4 cm. Overall, this study’s development and application of the HIME method added detailed insights to recent, transient ecogeomorphological change at the WLD, and demonstrated the potential of the new approach for accurately reconstructing past intertidal topographies and dynamic change. Keywords: delta; freshwater wetland; tidal wetland; Wax Lake Delta; ecogeomorphology; water-line method; marsh elevation; sedimentation; multiple stable states Remote Sens. 2020, 12, 1517; doi:10.3390/rs12091517 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 1517 2 of 31 1. Introduction Surface elevation is a geomorphologically and ecologically important variable in fluvial, floodplain, tidal, marsh, and delta environments. Sediment elevations high enough to experience regular subaerial exposure facilitate vegetation colonization. Enhanced vegetation growth and lateral vegetation expansion help stabilize sediments. Vegetation further aids sediment trapping at, and accretion to, the elevations suitable for its growth, closing the loop of a sediment-vegetation eco-geomorphological positive feedback. Sediment erosion between vegetation patches makes eroding areas less hospitable for vegetation colonization, while also providing part of the sediment source for nearby accumulation among vegetation, forming a second positive feedback loop, but in the opposite sense. Together, these two linked ecogeomorphological feedbacks are thought to create “stable attractors” of marsh elevation, i.e., elevations to which marshes will accrete, from slightly lower elevations, or erode, from slightly higher elevations. When sufficiently strong, these feedbacks lead to a wetland system self-organizing into discrete and spatially contrasting (i.e., patterned) vegetated and non-vegetated areas, or multiple vegetated and non-vegetated sediment platforms. Mathematically, the balance of accretion and erosion at a stable attractor should satisfy the condition that the vertical rate of elevation change over time is zero, i.e., dz/dt 0. Geomorphic stable attractors have therefore been ≈ diagnosed empirically by the resulting symptom of sediment accumulation at specific (attractor) marsh elevations and loss at intervening (erosional) elevations. This theory and various empirical cases of these phenomena were reviewed for tidal wetlands by Moffett et al. (2015) [1] and the feedback mechanisms thoroughly synthesized across fluvial and wetland settings by Larsen (2019) [2]. Although now fairly widely adopted to describe quiescent freshwater and saline tidal marsh settings, it is not yet clear if strongly advective fluvial settings, such as a young, prograding fluvial delta, can similarly exhibit eco-geomorphological elevation differentiation, especially over the short interannual to decadal timescales one might think to be dominated by strong exogenous forcing, such as hurricanes and major river floods [3–6]. For a young, rapidly prograding delta, island formation initially occurs subaqueously, governed by hydrodynamics and aqueous sediment transport. Flow expansion and deceleration where channelized flow transitions into a larger body of water induce sediment deposition, forming a mouth bar [7,8]. The location of the mouth bar, which sets the initial location for island formation, is strongly controlled by water velocity and depth and weakly controlled by grain size [8,9]. Field studies suggest that this jet deposit model initiates mounded island forms, with their thickest deposits at the upstream end and along the central island axis [10]. Flow splitting around the mouth bar causes channel bifurcation and levee development behind the bar [8,9]. Deltas exhibit strong morphodynamic coupling between surface elevation, water flow, and sediment deposition or erosion [11,12]. Bed or bar elevation controls the depth and frequency of inundation, which in turn affect bed shear stress and the amount and type of sediment available for deposition at a given location [13–15]. These processes are reviewed by Fagherazzi et al. (2015) [16], broader delta-forming processed by Ericson et al. (2006) [17], and specific mechanisms of delta sedimentation by Olliver et al. (2020) [18]. Once delta bars accrete to a level near or above mean water level, emergent plants are able to colonize the nascent islands. Wetland vegetation increases net mineral sedimentation by increasing bed roughness, slowing water velocity, and decreasing eddy diffusivity, thereby promoting particle settling and decreased resuspension [19–21]. Studies in non-delta marshes have correlated sedimentation rates with vegetation cover, though elevation and proximity to tidal channels are also factors [22,23]; results have been highly variable [24]. For example, if vegetation density, roughness, or height is sufficient to limit flow and transport across the marsh platform, vegetation can have an opposite, sediment-excluding, effect on wetland islands [18,25]. Cohesive and non-cohesive sediments may also exhibit contrasting behavior with respect to vegetation density and fluvial discharge: non-cohesive sediment accumulation may decrease with increased vegetation density and be insensitive to discharge, whereas cohesive sediment accumulation may increase with increased vegetation density and slightly decrease with increased discharge [25]. These conundrums further inspire this study to investigate Remote Sens. 2020, 12, 1517 3 of 31 whether a young, prograding fluvial delta setting, especially one with a high sand fraction among its sediment load, can self-organize according to multiple stable eco-geomorphological attractors, in reality, or whether at short, inter-annual to decadal timescales, exogenous forces will overwhelm local vegetation-sedimentation feedback mechanisms. If eco-geomorphological feedbacks are successful at stabilizing and helping to endogenously organize delta marshes, the combination of trapping and erosive feedbacks should lead to self-perpetuating sediment platforms at the discrete elevations of the eco-geomorphological stable attractor(s). A system may theoretically exhibit bi-stable states, i.e., simultaneously attract at different spatial locations within the domain of interest to one high and one low elevation; or a system may exhibit multiple stable states, with more than two stable attractors, each expressed simultaneously at different locations in the study domain. Note, herein we use the wording “multiple stable attractors” to refer to the co-existence of several discrete elevation platforms at different locations within one study domain, i.e., different marsh elevations reinforced by sediment-vegetation feedbacks among several vegetation species; this contrasts with the term “alternative stable states,”